DebugInfo: Permit DW_TAG_structure_type, DW_TAG_member, DW_TAG_typedef tags with empty file names.

Some languages, such as Go, have pre-defined structure types (e.g. "string"
is essentially a pointer/length pair) or pre-defined "typedef" types
(e.g. "error" is essentially a typedef for a specific interface type).
Such types do not have associated source location, so a Go frontend would
be correct not to associate a file name with such types.

This change relaxes the DIType verifier to permit unlocated types with
these tags.

Differential Revision: http://reviews.llvm.org/D8588

llvm-svn: 233200